1. Field of the Invention
The present invention relates to a high-intensity discharge lamp, also known as a metal halide lamp, for use in a vehicle headlamp, fog lamp etc. and other illumination devices. The invention more particularly relates to a mercury-free high intensity discharge lamp with high lumen output efficiency in the visible light wavelength, appropriate color rendering property and excellent discharge stability, enabling practical dimming of a headlight incorporating the mercury-free high intensity discharge lamp.
2. Description of the Related Art
In a conventional high-intensity discharge lamp such as a metal halide lamp, mercury has been used not only as a light emitting material, but also as a buffer gas in order to promote vaporization of other light emitting materials by increasing the temperature of a light emitting tube (arc tube) and to adjust lamp voltage of the light emitting tube. The lamp voltage can be understood as a voltage of the light emitting tube during steady lighting of the high intensity discharge lamp comprising the light emitting tube. Steady lighting is a state of lighting after a start-up or initial lighting period has finished. However, mercury is a toxic substance which has the potential to cause damage to the environment. Therefore, development of a light emitting tube which does not contain mercury is a long-felt need for manufacturers of high-intensity discharge lamps.
In another conventional metal halide lamp, a light emitting tube which comprises no mercury (referred hereinafter as xe2x80x9ca mercury-free light emitting tubexe2x80x9d) can be made by sealing a starter gas such as xenon (Xe) gas in the light emitting tube. The amount of sealed Xe gas corresponds to a few atmospheres or more at room temperature. Room temperature means substantially a normal, comfortable temperature. Thus, metal halides on a wall of a discharge chamber of the light emitting tube are vaporized by heat transmission from a xenon arc that has a high temperature and extends towards the wall of the chamber.
In the conventional mercury-free light emitting tube, major light emitting materials are metal halides which have similar thermodynamic properties to mercury. However, the conventional mercury-free metal halide lamp has different light emitting characteristics from the conventional mercury metal halide lamp. For example, in the conventional mercury metal halide lamp, if a dimming function is operated by decreasing input electric power to the metal halide lamp, the color of light emitted from the light emitting tube greatly changes because intensity of light emitted from mercury (having relatively high vapor pressure) is maintained while emission of light from other metals (metal halides) greatly decreases. On the other hand, in the conventional mercury-free metal halide lamp, if input electric power to the metal halide lamp is decreased, the color of light emitted from the light emitting tube changes in a smaller range, because light emission from each metal decreases keeping substantially the same ratio to all metals in the discharge chamber, and light emitted from each metal collectively constitutes the light emitted from the light emitting tube. However, the conventional mercury-free metal halide lamps have problems, some of which are described later in detail with reference to Japanese Patent Publications.
In yet another conventional light emitting tube capable of instant lighting, a starter gas including Xe gas is sealed in the discharge chamber in an amount of more than a few atmospheres at room temperature. A few times the rated current is supplied in an initial lighting period just after start-up of the light emitting tube. When the light emitting tube is started up from room temperature (referred hereinafter as xe2x80x9ccold startxe2x80x9d), electrodes disposed in the light emitting tube are heated to temporarily reach a high temperature, which expedites deterioration of the electrodes. Further, in a light emitting tube made of silica glass, electrodes which are made of tungsten are embedded in sealed portions of the light emitting tube located adjacent the discharge chamber. In this structure, mercury and metal halides creep and stay in a gap between the electrodes and the sealed portion when the light emitting tube was cooled by turning off the light emitting tube. Such mercury and metal halides located in this gap are instantly vaporized by a steep temperature rise on cold start of the light emitting tube, which may destroy the sealed portions of the light emitting tube where the electrodes are embedded. The lifetime of this kind of light emitting tube is substantially determined by the number of times cold starts that occur rather than the lighting hours. In cases where the metal halide lamp is used in devices which are frequently and repeatedly turned on and off, the lifetime of the light emitting tube can be greatly improved if the turn-off mechanism includes a dimming mechanism, i.e., number of times turnoff is decreased by replacing it with a certain dimming operations.
Japanese Patent Publication No. 6-84496 discloses a mercury-free high pressure metal halide discharge lamp capable of dimming. According to an embodiment of the patent publication, the high pressure metal halide discharge lamp comprises NaI 20 mg, ScI3 4 mg, and Xe gas which is sealed into a discharge chamber in an amount of approximately 8 atmospheres at room temperature. Rated electric power of the high pressure metal halide discharge lamp is 150W. If the rated electric power is decreased to 75W, the light color of the lamp is maintained, and a certain level of dimming without accompanying strangeness to a viewer is achieved. Further, the lamp voltage of approximately 90V is achieved by setting the multiplication factor of Xe gas pressure (atm.) and distance between the electrodes (mm) to be greater than or equal to 40.
According to results of the inventors trial and experiments, combination of NaI and ScI3 provides relatively good color rendering property and color reproducibility, i.e., color maintenance property before and after dimming, and high lumen output efficiency. However, the color of light obtained by the combination is rather greenish, and not pure white. According to testing and experiments, the light obtained did not fall within the scope of tolerance for white automobile light in the chromaticity diagram. Accordingly, usage of the high pressure metal halide lamp as a light source for illumination devices is limited depending on the required color rendering property for the illumination devices.
Lamp voltage is determined by the sum of voltage drop caused by electrodes and impedance produced by, for example, the electron scattering effect by metal atoms and produced by attachment of free halogens and electrons. Mercury greatly commits itself to areas of voltage because it has especially large collision cross section with an electron. According to the embodiment of the patent publication, no mercury is contained in the chamber of the light emitting tube. However, the light emitting tube achieved the same voltage as that of mercury-containing light emitting tubes. It is understood that vapor pressure of the metal halide was increased by operating the light emitting tube at a very high temperature. Since vapor pressure of metal halides is very high, it causes devitrification of the wall of the chamber and deterioration of electrodes due to reaction of the silica glass light emitting tube and the metal halides.
Japanese Patent Publication No. 11-238488 discloses a substantially mercury-free metal halide discharge lamp that includes a first halide with at least one metal selected from the group consisting of sodium, scandium, and a rare earth metal capable of predetermined light emission. The substantially mercury-free metal halide includes a second halide having relatively high vapor pressure and tendency of declination to emit visible light. The second halide includes at least one metal selected from the group consisting of aluminum (Al), iron (Fe), cadmium (Cd), zinc (Zn), tin (Sn), manganese (Mn), chromium (Cr), gallium (Ga), rhenium (Re), magnesium (Mg), cobalt (Co), nickel (Ni), beryllium (Be), titanium (Ti), zirconium (Zr), hafnium (Hf), and antimony (Sb). A rare gas can be sealed in a discharge chamber of the discharge lamp. The metal halide discharge lamp does not contain a substantial amount of mercury.
The second halide acts as a buffer gas, and produces the same lamp voltage as mercury. Efficiency of the lamp of the patent publication is improved by: 1) providing sufficiently high lamp voltage, which makes lamp current small, thereby preventing current capacity of the illumination devices incorporating the metal halide discharge lamp or circuit connected to the metal halide discharge lamp from increasing; and 2) reducing energy loss by electrodes. Further, it is also disclosed that a range of light color change is narrowed during dimming of the metal halide discharge lamp.
However, according to results of the inventor""s testing and experiments, the second halide emits light in an ultraviolet wavelength, which does not create lumen output in a visible light wavelength. In the metal halide discharge lamp according to the patent publication, although the lamp voltage takes an approximate value to that of the metal halide discharge lamp comprising mercury, lumen output efficiency in visible light wavelength of the conventional lamp free from mercury is smaller than the conventional lamp comprising mercury.
Further, depending on the additive amount of the second halide, halogen density during lighting is excessively increased, which tends to cause unstable discharge. In a state of unstable discharge, if the current and electric power are controlled to dim the light, unintentional extinguishment of the lamp (by discharge interruption) may often occur relatively soon after start of unstable discharge. Further, shading of ultraviolet light rays caused by the addition of the second halide is required depending on its wavelength and intensity.
Regarding usage of the metal halide lamp as a light source of an automobile headlight, a day-time running lamp (referred hereinafter as xe2x80x9cDRLxe2x80x9d) is required by regulations in some countries. The DRL provides light distribution in high-beam mode for illuminating a distant front area with smaller intensity than high-beam, while maintaining the color rendering property of light. However, a conventional metal halide lamp has not yet been used for DRL. The conventional metal halide lamp containing mercury is not able to operate dimming for light color change as described above. The conventional mercury-free type metal halide lamp has problems as described above when the dimming feature is operated.
Of course, not only when being used as a light source of an automobile headlight, but also when being used in various applications requiring to emit white light, it is preferable for the metal halide lamp to be capable of performing reliable dimming functions, i.e., adjusting light amount as required while maintaining color rendering property of the light, for efficient white light emission.
The present invention is intended to provide a high intensity discharge lamp which is substantially free from mercury and capable of providing high lumen output efficiency at visible light wavelengths and appropriate color rendering properties with superior discharge stability, enabling more practical uses for a high intensity discharge lamp with dimming function.
In order to resolve the aforementioned and other problems in the related art, the present invention can include a metal halide discharge lamp having the following characteristics. In a first aspect of the present invention, a metal halide discharge lamp comprising a light emitting tube, the light emitting tube comprising a discharge chamber formed in the light emitting tube containing no mercury, a pair of electrodes a portion of which projects into the discharge chamber, wherein the discharge chamber comprising a buffer gas, which also acting as a starter gas, of xenon (Xe) in 7-20 atmospheres at room temperature, and at least one kind of metal halide. The lamp has a positive resistance range in current-voltage characteristics relative to a varying input electric power, and in the positive resistance range, the light emitting tube is driven by an electric power which is equal to or smaller than a rated power supplied during steady state of lighting. The steady state of lighting is a state of lighting after a start-up lighting period has finished. In the steady state of lighting, the state of discharge is stable and the amount of luminous flux of the discharge lamp is stable as long as dimming operation is not performed. It is usual that rated electric power is supplied to the discharge lamp during the steady lighting period. In the metal halide lamp of the present invention, even if the input electric power to the light emitting tube is varied, flickering or sudden unintentional extinguishment does not occur, and the varying range of light color is narrowed.
In yet another aspect of the present invention, in the positive resistance range in current-voltage characteristics relative to a varying input electric power, electric power supplied to the light emitting tube is equal to or larger than 57% of the rated electric power supplied in the steady lighting period. By setting electric power to be in the above-described range, superior discharge stability which is appropriate for dimming light intensity of the headlight is provided.
In another aspect of the present invention, in the positive resistance range in current-voltage characteristics relative to a varying input electric power, total luminous flux varies in a range of 19-100% relative to luminous flux of the metal halide lamp during steady lighting. The range of total luminous flux provides a range of varying amount of light from the light emitting tube for use in the automobile headlight capable of dimming light intensity with stable discharge.
In another aspect of the present invention, in the positive resistance range in current-voltage characteristics relative to a varying input electric power, the input electric power varies in a range such that color of light emitted from the light emitting tube stays in a range of substantial white, enabling smooth dimming without accompanying great change of color rendering property which can be perceived to human eyes with strangeness. The substantial white means the following range in CIE 1931 xy chromaticity diagram.
xxe2x89xa70.345 yxe2x89xa60.150+0.640x
xxe2x89xa60.405 yxe2x89xa70.050+0.750x
The above range of chromaticity is consistent with a chromaticity range as specified in JEL 215 published by Nihon Denkyu Kogyo-kai for high intensity discharge lamps such as metal halide lamp of D2R type and D2S type used as a light source of an automobile headlight.
In another aspect of the present invention, the metal halides comprise at least sodium iodide (NaI) and scandium iodide (ScI3), thereby high lumen output efficiency in visible light wavelength is achieved.
In yet another aspect of the present invention, mole fraction of ScI3 relative to NaI is in a range of 0.10-0.43, thereby superior visible lumen output efficiency is achieved.
In still another aspect of the present invention, the metal halides further comprise indium iodide (I) in addition to NaI and ScI3. Mole percent of InI relative to all metal halides is in a range of 3-12 mol %, thereby the white light emission is achieved while limiting decrease of visible lumen output efficiency to an acceptable level as automobile light.
In another aspect of the present invention, the sum of molarities of all metal halides relative to an inner volume per unit of the light emitting tube is in a range of 30-100 xcexcmol/cc, thereby minimizing decrease of lumen output efficiency and change of chromaticity even after long lighting hours, and suppressing shading of light and unfavorable coloring to a predetermined color of emitted light by unvaporized metal halides.
In a further aspect of the present invention, in a period from start-up of the light emitting tube until it reaches steady lighting, electric power equal to or smaller than 300% of the rated power is supplied to the light emitting tube, thereby instant start-up of the light emitting tube is possible.
In another aspect of the present invention, the rated electric power of the light emitting tube is 35W, and lamp voltage of light emitting tube just after start-up is in a range of 15-25V. Further, lamp voltage of light emitting tube in steady lighting is in a range of 30-50V. In the above-determined range of electric power, the metal halide discharge lamp provides optimized electric property for use in an automobile headlight.
In another aspect of the present invention, the metal halide discharge lamp can be driven by direct current.
In yet another aspect of the present invention, wherein the light emitting tube has a range where impedance of the light emitting tube is equal to or smaller than 75xcexa9 in current-voltage characteristics relative to a varying input electric power, and the light emitting tube is driven during steady lighting by an electric power which is equal to or smaller than the rated power. Mole fraction of ScI3 relative to NaI is in a range of 0.05-0.43, thereby superior visible lumen output efficiency is achieved.
In another aspect of the present invention, the rated electric power of the light emitting tube is in a range of 10-50W, thereby size of the light emitting tube, which is appropriate for both instant start-up and the dimming operation, is determined.
In a still further aspect of the present invention, lamp voltage of the light emitting tube with rated electric power in the range of 10-50W is in a range of 20-65V in steady lighting, thereby appropriate voltage and current for dimming operation of the light emitting tube is obtained. Electric power supplied to the light emitting tube during steady lighting varies in a range of approximately 40-100% of the rated electric power during steady lighting, thereby discharge without unintentional extinguishment during the dimming operation can be achieved.